A headphone is a pair of small loudspeakers that are designed to be held in place close to a user's ears. However, the soundstage given by existing headphones is usually not very spacious or not realistic. In order to let the user experience a more natural sound, many methods for adjusting audio frequencies have been applied to the existing headphones. In one of the methods, a small piece of sponge is used for producing a certain distance between a sound-producing unit and the ear, such that the ear not only hears sounds emitted from its adjacent sound-producing unit but also hears sounds emitted from its opposite sound-producing unit. In another method, the sound-producing unit is designed to be close to the ear and is arranged forward. In another method, sounds are reproduced through circuitries. The above-mentioned methods, however, are complicated and have down side.